A. Field of the Invention
This invention relates to the field of backup electrical power sources and in particular to electrical power sources using a thermopile.
B. Background Art
Electric power transmission towers offer a very promising, already existing way to distribute optical fiber communications. However, this method suffers from a major limitation. Fiber optic systems require repeaters to regenerate signals in order to communicate over large distances. These repeaters contain active electronics which must be powered from an external source.
Several sources of power for these optical repeaters have been developed but each has major drawbacks. For example, a dedicated power distribution line may be constructed to power the optical repeaters. However, these power lines have costs of several tens of thousands of dollars per mile, making a fiber optic communication system of any significant distance powered this way economically prohibitive. Furthermore, use of this type of power source does not provide any backup when supply from the power line is interrupted.
Optical repeaters may be powered by supply conductors within the optical cable itself. This technique has been used in undersea optical cable installations where other forms of power are unavailable. This significantly increases cable costs, especially if a second set of power conductors is required to increase the reliability of the supply.
Alternate energy sources such as photovoltaic arrays or wind turbine generators may be used to eliminate reliance on power distribution lines. Unfortunately, these sources are highly variable in the best of locations and may be completely unsuitable in many locations. Even where solar energy and wind are available in abundance, provisions must be made to supply energy for the repeater when these sources are unavailable. Battery backup systems are useful, but they have high initial expense and must be frequently maintained to insure continued reliability.
Also, thermoelectric generators may be used. Thermoelectric generators using gaseous fossil fuels have been commercially available for many years as, for example, the fossil fuel thermoelectric generator produced by Global Thermoelectric Power Systems Ltd. These generators provide a direct current output by heating a junction of dissimilar conductor materials through combustion of a gaseous fuel. Use of this principal is particularly attractive because these generators have few moving parts, long service life, and proven reliability. They also provide protection from the input transient phenomenon which may occur on the utility distribution line since use of the distribution line is avoided. However, if this method of heating the thermoelectric junction were used alone, the tanks storing the gaseous fuels would have to be refilled or replaced periodically causing great inconvenience, since many distributed loads may be in isolated areas.
A way to obtain power for small distributed loads from the utility transmission line at low cost was taught in Supplying Fixed and Stroboscopic Light Beacons from the Overhead Ground Wire on 735 kV Transmission Lines, by R. Blais, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-99, No. 1 Jan./Feb. 1980. In power transmission lines, there is normally an overhead ground wire which links the tips of the towers to protect the line conductor bundles from lightning. The wire is normally grounded and is not used to carry electrical energy. However, a certain amount of energy is coupled to this overhead ground wire. If a predetermined length of the ground wire was isolated from the rest, it was possible to tap this energy without affecting the primary function of the wire. The power which may be tapped by this method was normally unused and wasted. The isolated section of ground wire was coupled through the primary of a transformer to ground.
Blais teaches the use of energy obtained in this manner to power the beacons on towers which are required for the protection of aircraft. In the same issue of the IEEE Transactions, Berthiaume teaches the use of the same method of tapping power to energize microwave network repeater stations in remote areas. Microwave Repeater Power Supply Tapped from the Overhead Ground Wire on 735 kV Transmission Lines. However, both Blais and Berthiaume have the disadvantage that they are still dependent upon the energy normally carried by the utility transmission system to capacitively induce energy in the section of overhead ground wire which is used to power a distributed load such as a beacon or a microwave repeater.
The energy normally carried by the utility transmission system is interruptable and unreliable for a variety of reasons. Overvoltage conditions can damage repeater circuits. Storms, floods, or lightning may damage the towers or the power transmission lines themselves. Additionally, system overloads, equipment failure and human error may cause blackouts or brownouts. Any event which interrupted power transmission would, of course, interrupt communication links dependent upon the power for energizing repeaters.
A way to increase power supply reliability is to provide a backup source which is activated when a primary source fails. One method of switching from a primary power source to a backup is described in U.S. Pat. No. 3,636,368, issued Jan. 18, 1972 to Joseph B. Sia, titled Transfer Switch and Generator Control Means, and New and Improved Method of Operation Thereof. Sia teaches the automatic transfer of a load from the power line to a backup generator. When the line voltage sensor detected a voltage below a predetermined threshold, the generator was activated. Disadvantages here were the cost of the power delivered to the load from the power line, the cost and unreliability of the generator, and interruption of power due to generator startup time.
Other patents of interest found in a brief patentability search of the present invention include: U.S. Pat. No. 4,161,650, which relates to a self-powered fiber optic interconnect system; U.S. Pat. No. 3,670,175, which teaches supplying electric energy alternately from a voltage line and a rotating electric generator; U.S. Pat. Nos. 3,614,461, 3,489,346, and 3,389,303, which relate to circuitry for control of a power supply from a primary AC source to a substitute source upon failure of the primary; and U.S. Pat. No. 3,506,841, which relates to the use of propane vapor in a power pack for long-term unattended operation.
It is, therefore, an object of this invention to provide a reliable source of electrical energy for distributed loads such as repeaters for a fiber optic communication system.
An additional object is to provide a thermoelectric generator with an auxiliary source of thermal energy.